Carbon Fibre Commuter Bike

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Design Objectives
To investigate a carbon fibre framed non folding commuter bike.

Whilst riding my Giant halfway to work everyday, I often think how the design is compromised by the hinge in the centre. This makes the frame flexible and it also squeeks terribly. I travel through a lot of parks going to work along gravel tracks, which the bike wasn't really designed for. As I don't use it on public transport, a nice non folding compact bike would be ideal for the 20 mile round trip. I have always wanted to build a carbon fibre frame, indeed this desire stems back to my recumbent building days nearly 20 years ago. I even built a few mockups in foam, but I never had the opportunity to give it a go.


13/11/2012 Plodding On
Not many posts recently due to leaking roofs at home and a few other things going on, but work is progressing at a steady pace. I have been waiting on a headset bearing, so I can draw it up and finalise the fork, headset bearing and handle bar position. I have also detailed up the rear dropouts and am still working on the exact shape of the frame. I managed to locate some blue foam on ebay and it was local to my London digs so this is now sitting in my garage awaiting use. I have also ordered some aluminium tube of the correct diameters for the front of the bike. I will salvage the bottom bracket and seat tube from the GT frame. Still a lot of detailed work to do and then to double check everything before making a start. I have plenty of time as most of the work needs to be done outside and its too cold at the moment!

27/10/2012 Full Circle
More redesigns and I am now back to a carbon fibre over foam frame. Again looking on the net I have found a Shimano Alfine rear hub which has 8 gears internally. These are around 120, so affordable when you consider you dont need a conventional rear hub, rear gears or derailleur. This would also simplify the rear dropouts, in fact they could probably all be done in carbon fibre. I spent today designing various frame configurations and kept redoing them until I got one that looked about right. It uses simple flat sections with radiused corners, so should be fairly easy to knock up in foam. It still needs lots of work to check chain clearances, headset diameters etc. but this looks like the primary candidate... at least for today!

25/10/2012 Material Change
The sheet metal bike would have problems, notably that if you fall off the bike, the metal edges could do you quite a bit of damage! More web searching has thrown up some wood bikes and in particular plywood sheet used vertically. Here is my spin on the wooden theme, using reprapped blocks.

The blocks have a form where they have three tongues to clamp the wood edges so they spread the load on the wood when bolted through. If printed with 100% fill they would be very strong. The wood can be layered and glued over a former, so giving the correct bends in the Y axis to go around the rear wheel and give correct crankset clearance. In addition, I can laminate the wood if needs be with fibreglass or carbon fibre should the 12mm plywood prove too weak (Which I don't think it is). I still need to finalise the rear dropout design. The cables would  be hidden between the sides and the frame is smaller than a sheet of A0 paper. This means I can print off a plan easily during construction.
The reprapped blocks on the headset seem suitable for mounting lights and speedometers and for routing the cable ends. I still think I need additional blocks possibly for a rear V brake and also one at the middle of the vertical seat tube. This will mean I need a long seat post, but these are readily available. As I look at the frame more, I can see all sorts of bits that I could reprap, such as a guard to stop chain slap, a bottle rack or a rear light mount. It might even be possible to retro fit a front
derailleur clipped onto the edge of the wood. I like this design as its all bolted together so should I get a failure (If i am not in hospital!) I can remove it and do a redesign. I am concerned about the bottom bracket, so this may need more thought.

24/10/2012 Further rethinks!
More thoughts on the frame and more research on the net threw up bikes made of plywood sheet, but none of flat sheet metal. I found one that was fabricated with riveted box sections. Here is what I have come up with after another few hours on CAD.

Sheet aluminium supported by solid reprapped blocks (Shown in dark grey) bolted through. Other thoughts are to use Hexel sheet which is a honeycomb structure sandwiched between aluminium or carbon fibre sheet. It is used in aircraft and race cars.

23/10/2012 A rethink
As the carbon over ABS was a technical dead end it's time to consider something different. After about an hour on CAD this is where I am at.

Using three tubes to connect at the front seems to disguise a long headset and would enable the use of more conventional handlebars. Trouble is it doesn't look very radical. Maybe if I use ABS lugs and aluminium tubes as done on The Andrew Leinonen Bike. Instead of aluminium tubes I could use carbon fibre ones. Using round tubes also gives the chance to use the folding concept on Philip Crewes Bike which is really neat, but the S & S couplings are around 225 each.. Ouch!

I have decided to purchase a few parts to keep things moving along. Its a gamble, but I really need to start finalising the dimensions if this thing is ever going to get done. With that in mind I have bought a
FSA Orbit XL 2 headset as I have on the Salsa bike. I don't fancy stripping that one down to measure up the bearings, so I have bought another. I also spotted a Brompton crankset so I have bought that as well. It has a single 50 tooth chainring. This seems about right as my Giant Halfway has a 48 tooth ring and I always seem to be in 6th or 7th gear. The extra two teeth should give me a little more headroom. 

22/10/2012 Getting Technical!
At work today I measured the samples that I had created over the weekend.

All samples were printed using 0.5mm layer height. 60mm x 40mm x 70mm high, 15mm external corner radius.

Sample 2
3mm wall width 2 perimeter layers 10% infill (This actually fills the section)
Estimated print time (SFACT) 1h 59m 42s         Actual Print time 1h 48m 18s
Weight 29g         Giving a weight of 414.285 g/metre of tube

Sample 3
5mm wall width 2 perimeter layers 10% infill
Estimated print time (SFACT) 1h 57m 06s         Actual Print time 1h 49m 48s
Weight 29g
        Giving a weight of 414.285 g/metre of tube

Sample 4
5mm wall width 2 perimeter layers 5% infill
Estimated print time (SFACT) 1h 49m 01s         Actual Print time - Not Recorded
Weight 26g
        Giving a weight of 371.428 g/metre of tube

Lessons learned.
A narrow wall section doesn't always lead to a lighter part.
5% infill produces a strong accurate part and is lighter, but not by much, however it does add up, if the frame has say 2 metres of tube, I could save 85g. Even so, it doesn't look like I can build a light frame this way. 741g just for the 2m of skeleton, this doesn't include aluminium bottom bracket or headset, or carbon fibre outer.

An outer layer of carbon 2mm thick would weigh 
577.699g/m (even at a good 1.6 relative density)

Total weight 371.428g/m + 577.699g/m = 949.127g/m

Aluminium tube 1.5" OD (38.1mm) x 0.9mm wall x 1000mm long would weigh 283.987g
(T6 aluminium has a relative density of 2.7)

Carbon Fibre tube
1.5" OD (38.1mm) x 0.9mm wall x 1000mm long would weigh 168.289g
(Bought carbon fibre tube has a relative density of 1.6) (ref

Conclusion, Aluminium tube with reprapped lugs would be much lighter than a complete reprapped frame covered in carbon. A carbon tubed frame with reprapped lugs lighter still.

21/10/2012 Developing a lightweight Skeleton
This weekend I have been printing of some short 70mm sections of the proposed frame using several different fill ratios and changing the number of perimeters. I now have 3 samples which seem quite strong and light. These will go to work with me tomorrow so I can weigh them on the accurate scales which they have. Its strange as thick walls can be lighter and stronger as its dependant on how the SFACT software determines the fill pattern. I am working on a 5% fill 2 perimeter layer 5mm wall bieng the best. Looking at my other bikes it seems that a 40mm height x 60mm wide box section is perhaps a bit too large, so this may change. This is good as it would mean the reprapped skeleton may be even lighter. It won't be as light as foam, but may be acceptable. This project is becoming more involved, so I haven't yet discarded the idea of using aluminium tubes with reprapped lugs or indeed a foam cored one. I need to keep designing more until I am happy that I will be building a bike that is worth the effort and not a real heavy lump.

16/10/2012 Design and redesign
I am now on my 8th design of frame. I cannot seem to make the long fork designs look any good, so I have done a new design with a 20" fork and long head tube. I have also added some more generous fillets on the tubes. This makes it look much more proportioned. With the longer head tube, this raises the issue of locating a set of 20" forks with a long steerer tube. This is a problem, but a good detailed search on the net highlighted a possible solution, namely to use a 1" steerer tube, with 1 1/8" long 6061 T6 aluminium tube glued on the outside. This sounds a bit strange, but it is used on recumbent bikes without problems. The tube would be glued with good epoxy and over a very large length (200mm). I could then use 1 1/8" headset bearings top and bottom. As for the handlebars, trials bikes often use 3" riser bars and these might be just enough to get the bars to my desired height.

The bike frame construction has also recieved some thought. I have some PU foam at work and this could be used for the frame core and would require a lot of shaping....This requires some artistic skill which I have a shortage of. I am now considering a reprapped skeleton done in short sections, glued together, then wrapped in carbon fibre tape. This would be heavier and printing large sections is tricky, but by using a 1mm wall, and slotting the outside regularly I may be able to control warping and keep the weight down to close to that of a foam core. In addition I can put internal details to hold brake and gear cables and print parts to locate the aluminium sections of the frame exactly. To get the frame straight when gluing the sections together I may print the frame in two halfs and assemble them on a flat board marked out with the frame shape and sections. Whilst on the flat board, a few layers of carbon would make the frame halfs rigid enough to remove and join together. I need to do a few trial sections to see if this is feasible. It looks like it will soon be time to commit some money for parts or abandon the project.

My 15 girls bike looks quite good now I have had a good look. It has alloy rims and good tyres which are worth the price alone.

14/10/2012 Tall oaks from small acorns grow!
Last night on ebay I spotted a bike for sale for 15. As it was on my way down to London today, I purchased it and picked it up this afternoon.

Ok it doesn't look much like a carbon fibre show winner, but it has some useful bits on it such as tyres, wheels, and dropouts. It might even be possible to use the handlebars, as the ones on a commuter bike are a little narrower than mountain bike ones. Even if I dont end up using much, it will give me an idea of the component sizes for CAD modelling.

On the design front, I showed my initial design to a few people at work and feedback was quite negative, which confirmed what I thought. This weekend I have done a few more frame designs and the following one seems the most pleasing. I like the dorsal fin look of the seat stem, which is a feature I will be keeping. It also means I can fasten a seat clamp with screws into the frame. This is another potential weakness avoided. The length of seat tube also means that I could do this all in carbon fibre and avoid an inner metal tube. I need to work on the frame tubes and try to incorporate some larger radii and curved organic form. I did a version in ovalised tubing and it looked worse, so the square section seems to be the best, and of course is more striking. The Ibis bike company does some nice organic curved frames, so I might have a look at those a little closer for inspiration.


A search on the internet has revealed many articles and videos of home made carbon frames. It looks like something I could attempt at home, but probably during next summer when the garage is warmer and more inviting.

Some designs that have inspired me are as follows. A nice but expensive bike with a mono strut front fork like my Giant Halfway.

The Lexus concept bike. Realy nice curves, but the frame design looks like it may snap in half if not laminated properly.

The bonobo wooden bike. Really sleek lines and some nice headset and dropout details.

A few nights designing on Catia I have come up with the following. Its very ugly with square tubes, but at the moment I am just defining the hard points of the frame. I would like the headset up high just under the handlebars, but have 20" wheels. I have noticed that junior MTB bikes have 20" wheels with gears. BMX bikes have one gear and have unusual hardware and hubs, so they are of no use. To raise the headset I need long forks, so these will be mountain bike 26" items. This should give the front end an unusual look, but make fitting a rim brake a challenge! I will also need to raise the handlebars somehow, but extenders are available quite cheaply. The crankset will have a 48 tooth front which I have modelled up. Some additional CAD models have been employed downloaded from GrabCAD

As for construction, I am looking at carbon fibre wrapping over foam, or possibly carbon over reprapped ABS sections. I like the idea of using reprapped sections from my machine as this will save a lot of sanding and sculpturing, just print an open thin frame in sections, glue it together and then wrap it up in carbon bandage. I want to avoid vacuum bagging as this is messy and wasteful and I would also like to use 50mm wide carbon fibre tape. Reading up on other projects, the main issue is interfacing with the hard points, such as dropouts, head tube and bottom bracket. This is why the bonobo bike appeals to me. I could bolt the parts to the frame and ensure a good fix to the carbon fibre. perhaps there is a half way, bolt / rivet, then lay up over the top.

I don't want a perfect looking frame and would actually like a homespun rough matt look to the bike. A sort of Rat Rod commuter would be nice.